1. Field of the Invention
The present invention relates to a magnetic particle type electromagnetic clutch.
2. Prior Art
FIG. 4 shows a cross-sectional view of a prior art magnetic particle type electromagnetic clutch. A drive shaft 106 is driven into rotation by a drive source (not shown) so that a drive element 107 mounted on the shaft rotates in a unitary motion with the shaft 106. When an excitation current is run through an excitation coil 102 housed in a stator 101, a magnetic flux.PHI. is generated as shown in FIG. 4. With the aid of the magnetic flux.PHI., magnetic particles 110 forming part of the magnetic path are magnetized to make up a chain-like connection between a driven element 109 and the rotating drive element 107 so that the two elements 107 and 109 attracts each other to become together. Thus, a drive force is transmitted to the driven element 109 so as to drive the driven shaft 111 into rotation. When the current through the excitation coil is shut off, the magnetic flux is diminished so that the magnetic particles 110 are free from chain-like connection leaving the driven element 109 free from the drive element 107. The torque transmitted from the drive element 107 to the driven element 109 is substantially proportional to an amount of current through the excitation coil 102; the torque is controlled at will by varying the current. Therefore, the variation in manufacture of the clutch and the deterioration of magnetic particles with age adversely affect current-to-torque characteristics resulting in insufficient torque control. For sufficient torque control, a torque detector is required in addition to the above-described magnetic particle type electromagnetic clutch, resulting in a complex and expensive clutch construction.
FIG. 5 shows a film winding device where such an electromagnetic clutch is used. In the figure, the drive force of a motor 115 is transmitted via a magnetic particle type electromagnetic clutch 116 to a driven shaft 111. The rotation of driven shaft 111 is transmitted to a takeup shaft 120 via a belt 119 to wind up a film 121. Rollers 126 and 127 give a tension to a tension-detecting roller 125. The tension detectors 123 and 124 detect the tension of the film 121 via the tension-detecting roller 125. The outputs of tension detectors 123 and 124 are supplied to a controller 128 which controls the current being supplied to the electromagnetic clutch 116. This kind of construction requires the independent tension detectors 123 and 124 and the rollers 122, 126, and 127 for giving a tension as well as the overall construction is complicated and expensive. In addition, a space for mounting the tension detectors is often not available.